The emergence of primary lithium-sulfur dioxide batteries represents a significant advance in battery technology due to their outstanding performance and also because they represent the first successful use, in a nonreserve, primary cell, of a strong oxidizing agent in a stable, intimate contact with a strong reducing agent. The Li/SO.sub.2 system is now an accepted member of the battery family with a growing range of applications. Although applications of greatest interest involve primary Li/SO.sub.2 cells, the development of both reserve type and rechargeable cells has also received attention.
The usefulness of the Li/SO.sub.2 system stems primarily from its outstanding performance. It has about twice the energy density of conventional alkaline systems, a very high rate capacity and a low temperature performance which is better than that of any other primary battery. In addition, its high temperature shelf-life is excellent and the battery has been shown to have good capacity retention even after 5 years of storage under ambient conditions.
The battery comprises an anode, a cathodic component and an electrolyte. The anode is formed from lithium metal or its alloys with or without a support which, when present, may be in the form of a nickel coated screen or the like. When the bare lithium is initially exposed to the electrolyte mixture containing SO.sub.2, it reacts spontaneously to form an insoluble film on its surface. The morphology of the film permits lithium ion transfer and inhibits direct chemical reaction without further separation of the Li and SO.sub.2. The electrolytic component generally comprises a lithium salt, such as a lithium halide, carbonate or hexafluoroarsenate in organic solvents which also contains the SO.sub.2 liquid. Typical organic solvents are propylene carbonate, acetonitrile, dimethylformamide, dimethylsulfoxide and the like. The cathodic component of the battery comprises the sulfur dioxide (also acts as an electrolytic co-solvent) as the active positive material and carbon electrode which both catalyzes the discharge of sulfur dioxide and carries the electronic current to the current collector.
The cell discharge reaction involves: ##STR1## During the discharge of the primary Li/SO.sub.2 cell, insoluble lithium dithionate is deposited at the carbon electrode.
The present invention is directed to an improved carbon electrode useful in a Li/SO.sub.2 battery. The subject carbon electrode can also be effectively used as a component in a reserve type lithium-liquid depolarizer battery in which the depolarizing liquid is, for example, SO.sub.2, SOCl.sub.2, SO.sub.2 Cl.sub.2 and the like.
At present, one of the most limiting factors in providing an effective Li/SO.sub.2 battery resides in the cathodic carbon electrode. The carbon electrode, although not taking an active part in the redox reaction of the battery, plays an extremely important part in achieving a highly effective battery. The electrode must be highly conductive and, therefore, must be a product having very high percentages of electrically conductive material. In addition, the electrode should have a high capacity to effectively store the battery discharge precipitant, lithium dithionate. The electrode should be highly porous and have substantially uniform porosity throughout, including the electrode's thickness. It is also highly desirable to have the carbon electrode in the form of a free-standing sheet having rheological properties which permit it to be manipulated and processed into various battery configurations such as plates, jelly rolls, accordian shape and the like without causing defects in the carbon sheet. It is desirable to have the carbon electrode produced in a non-aqueous system to prevent corrosion within the electrode (of the current collector) and the system (of the anodic metal). Finally, it is desirable to have a carbon electrode which is capable of being readily formed in a manner which is not labor intensive.
The carbon electrodes presently used in Li/SO.sub.2 batteries are constructed from a porous mass of carbon black with a polyfluorocarbon polymer binder. The binder is normally present in from 5 to 20 percent based on the total electrode composition. Normally aqueous slurries of the carbon and binder are prepared and then formed into sheets by calendering or doctoring the slurry into plates. A current collector, such as an expanded aluminum screen is pressed into the sheet, the aqueous solvent then removed by drying the product at elevated temperatures and the binder is then cured. Several drawbacks reside in the polyfluorocarbon - carbon electrode. The product exhibits poor integrity with elements flaking off when in the final dried state. Further, flaking of carbon particles may cause contamination of the equipment and internal shorting of the battery. The presently known electrodes do not exhibit sufficient rheological properties to readily permit winding of the electrode into narrow diameter cells. The porosity and uniformity of the final electrode product are poor due to the required mode of drying which inherently leaves pinholes in the product and causes greater porosity at the electrode surface then in its core providing a pore size distribution which is very large. The larger pores do not provide the needed surface area to volume ratio for proper utilization of the carbon and sufficient capacity for precipitation of the discharge product, lithium dithionate. Further, voltaic corrosion of the aluminum current collector is observed and is attributed to the presence of water during the formation of the electrode.
The polyfluorocarbon bound carbon electrode is hard to fabricate into a thin, uniform sheet product. The process of forming is labor intensive (partially due to the large degree of inspections required) and costly (partially due to the irreproducebility of the product).
It is highly desired and the object of the present invention to produce carbon electrode suitable for use in a Li/SO.sub.2 system which has a very high carbon content, has a uniform porosity and a narrow pore size distribution, exhibits a high degree of integrity and stability, has good rheological properties to provide a free standing sheet capable of exhibiting a high degree of flexibility, and which can be readily formed without the aid of water in a labor and cost effective manner.